The present invention relates to a gas turbine of the type in which an air/fuel mixture is reacted in a catalytic combustor.
Gas turbine systems have been previously proposed in which an air/fuel mixture is compressed by a compressor, and then reacted in a catalytic combustor. For example, U.S. Pat. No. 4,754,607 describes a self-contained energy center or cogeneration system which converts chemical energy into mechanical, electrical, and heat energy. The fuel, preferably a gaseous fuel such as natural gas, is mixed with air in a mixer, and then the mixture enters the compressor. The compressor compresses the air/fuel mixture and outputs the compressed mixture to the cold side of a heat exchanger in which the mixture becomes heated. The heated, high-pressure mixture is then delivered to the combustion chamber of a catalytic combustor. The resulting products of combustion are directed to the inlet of an expansion turbine mounted on the compressor shaft. After powering the turbine, the hot combustion gases are directed through the hot side of the heat exchanger, whereupon those gases supply the heat which is transferred to the cooler air/fuel mixture passing through the cold side of the heat exchanger. The still-hot combustion gases exiting the hot side of the heat exchanger are delivered to heat-utilizing devices such as a hot water heater. Meanwhile, the turbine drives an electric generator mounted on the compressor shaft for producing electric power.
During start-up of the system, the combustion chamber of the catalytic combustor is too cold to combust the particular air/fuel mixture used during steady state operation (e.g., natural gas). Therefore, there is provided a separate preheat burner disposed in the conduit which connects the outlet of the turbine to the hot side of the heat exchanger. The preheat burner is supplied with fuel to create combustion gases. Those gases are then supplied to the hot side of the heat exchanger for preheating the air delivered to the cold side of the heat exchanger from the compressor (which is being motored-over during start-up). The air preheated in the cold side of the heat exchanger is then conducted through the catalytic combustor to heat the latter. Once the catalytic combustor has been sufficiently heated to support combustion of the steady-state air/fuel mixture, the preheat burner is deactivated, and the steady state air/fuel mixture is fed to the compressor.
The above-described system exhibits certain shortcomings, especially as regards the start-up or pre-heating operation. In that regard, the start-up procedure requires that heat be transferred from preheated air to cold air traveling through a heat exchanger, and a subsequent transfer of that heat from the air to the catalytic combustor. Due to the temperature limits of the materials in the heat exchanger, and the mass of the heat exchanger which must be heated, that procedure is highly time consuming, requiring that the high energy-consuming starter motor be driven for a relatively long period, e.g., two minutes or longer, thereby considerably reducing the efficiency of the system.
It has also been proposed to provide a pre-heat burner upstream of the catalytic combustor. However, that arrangement involves a number of shortcomings. Firstly, the arrangement is inefficient, because it requires that all of the air or air/fuel mixture must be heated to the light-off temperature. Secondly, the transition from the pre-heat state to the steady state is difficult, since the pre-heat burner includes an open flame, and the ignition of the steady state air/fuel mixture may flash back into the pre-burner, causing the pre-heat burner or the catalyst to overheat.
In addition, if the pre-burner fails to light immediately, locally high concentrations of fuel may be introduced into the catalyst, which can cause overheating when the pre-burner thereafter ignites.
Finally, if the pre burner is positioned in the main flow it may fail and damage the catalyst and introduces parasitic pressure losses in other modes than start-up. If it is placed outside the main flow, complicated and costly valving must be supplied to direct the flow through the pre-burner during start-up and through the catalyst in all other operating modes.
Placing the pre-heat burner downstream of the catalytic combustor solves some of the above mentioned problems, but still requires that the gaseous fuel be compressed separately, or that an alternative fuel be used for start-up. Also, if a downstream burner is placed in the main flow from the catalytic combustor, it must withstand the high temperatures exiting therefrom in operating modes other than start-up, which increases the problem of failure and damages to the turbine and also introduces parasitic pressure losses, decreasing efficiency. If such a burner is not placed in the main flow, complicated valving must be provided to direct the flow through it during start-up, and isolate it during other modes of operation.
Many techniques for achieving light-off have also been proposed in "Fuel Injector, Ignition, and Temperature Measurement Techniques for Catalytic Combustors," Proceedings Fourth Workshop on Catalytic Combustion Cincinnati, Ohio, May 1980. Most of these methods consist of placing an ignition source upstream of the catalyst, which has the risk of overheating local areas, which can lead to premature failure of the catalyst or the substrate due to overheating and/or thermal shock. Others of the methods involve electrical resistance heating of the combustor air, which requires very large quantities of electrical energy, leading to impractically large and costly batteries when grid power is not available. Also discussed is the introduction of hydrogen of the catalyst, by direct injection or by releasing hydrogen, trapped as a hydride, by electrolytically heating a structure placed upstream of the catalyst. However, the hydride storage structure has the disadvantage that parts of the structure may separate and move downstream into the catalyst, blocking portions thereof and leading to failure or excessive emissions. Direct injection of hydrogen is proposed, but dismissed because it is "dangerous, difficult to handle," and "unattractive to potential users". To this should be added the fact that storing sufficiently large quantities of hydrogen for prolonged operation is both costly and perceived as dangerous. Further, if the hydrogen is not produced or stored at sufficient pressure for injection, it would be necessary to provide a gas compressor for generating such pressure which is expensive and dangerous.
Therefore, it would be desirable to provide a safe, simple, and economic way of pre-heating a catalytic combustor and which does so more quickly than reliance on a heat exchanger.
It would also be desirable to provide such a pre-heating technique which does not depend on thermal energy from a burner upstream or downstream of the catalyst, and which overcomes the difficulties associated with direct hydrogen injection.